Electrochemical devices have wide application areas in the economical and societal most relevant fields such as energy, health and environment. Just to name a few, applications of these devices include solar cells and solar hydrogen generation by water splitting for solar energy harvesting and storage; biosensors for medical diagnostics, environmental monitoring and food control; and batteries, supercapacitors and fuel cells for energy storage and supply.
All these electrochemical devices and their applications rely on efficient electrochemical electrodes characterized by high catalytic activity of the electrode material surface together with large surface area. Many widely used electrode materials, such as metal oxides and group IV and group III-V semiconductors, have a crystal structure with crystallographic planes of different catalytic activity due to different surface energy and chemical bond configurations. For high electrode efficiency, the exposure of crystallographic planes with high catalytic activity for a certain electrode material together with large surface area of these planes are required.
Patent application JP 2009-019233 A discloses a method of fabrication of electrodes that broadly comprises the steps of high-rate depositing a material onto a surface of a substrate, detaching the obtained deposit in the form of powders from the substrate; and transferring the powder onto a conductive support, e.g., a copper foil; to produce electrodes for batteries.
A possible way to maximize surface area is to develop nanostructured electrode materials either by utilizing self-assembled isolated/colloidal nano- and microstructures or by utilizing self-assembled electrode surface nano- and microstructures such as nano/microwalls, nano/microflakes or nano/microwires, rods, columns. This approach, however, inherently leads to exposure of surfaces with low or no catalytic activity due to minimization of the surface energy in the formation process.
Among the cited semiconductors, so far Indium Gallium Nitride (InGaN) has allowed the production of electrodes with the highest efficiencies per surface area, in particular when prepared in form of layered material and related heterostructures exposing the catalytically active crystallographic c-plane. Independently, catalytic activities can be improved by doping or (co/electro-)catalyst coupling where there is no increase of surface area. The best results have been obtained with epitaxial InGaN material with surface InN quantum dots (QDs) on a sapphire and silicon substrates. QDs are islands of the material with size of a few nm in all three spatial directions, capable to confine the charge carriers (electrons and holes) in an extremely limited space; this arrangement gives rise to novel or enhanced optoelectronic properties. InN/InGaN QDs and their applications in the fields of electrochemical reactions in general have been described in several papers, such as “An InN/InGaN Quantum Dot Electrochemical Biosensor for Clinical Diagnosis”, Naveed ul Hassan Alvi et al., Sensors, 2013, 13, 13917-13927; and “Electrocatalytic oxidation enhancement at the surface of InGaN films and nanostructures grown directly on Si(111)”, Paul E. D. Soto Rodriguez et al., Electrochemistry Communications, 60 (2015) 158-162.
The paper “Indium-related novel architecture on GaN nanorod grown by molecular beam epitaxy”, Y. H. Kim et al., Chemical Physics Letters, 412 (2005) 454-458, describes the production of GaN free-standing nanostructures; no particular morphology of these nanostructures, in relation to the crystalline lattice axes, is mentioned.
Structures produced by epitaxially growing films on silicon (111) substrates are known in the art, e.g., from patent application JP 2004-319250 A; this document describes the production of InGaAs, p-doped layers.
A general problem encountered with these materials is the potential limitation, for geometric/structural reasons of the substrate. Mainly the substrates inhibit the increase of surface area of the electrode active material on which the QDs are produced. Besides, the performance of these materials, albeit satisfactory, is still susceptible of improvement. The problem is, therefore, how to realize an electrode with increased surface area, exposing surfaces with high catalytic activity of the electrode active material.
It is therefore an object of the present invention to provide an electrode having, as the active element, fragments of InGaN and related heterostructures with increased surface area and exposing surfaces with high catalytic activity.